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Current Research and Scholarly Interests

My work primarily involves characterizing the elusive endothelial progenitor cell (EPC) in adult tissue; its origin, expression profile and function. This work is being accomplished using surgical models of parabiosis and ischemia, flow cytometry, single cell microfluidics and high-throughput imaging. Further, the work is extended to look at dysfunction of EPCs in diabetes and aging.

Abstract

Multipotential stromal cells or mesenchymal stem cells (MSCs) have been proposed as aids in regenerating bone and adipose tissues, as these cells form osteoblasts and adipocytes. A major obstacle to this use of MSC is the initial loss of cells postimplantation. This cell death in part is due to ubiquitous nonspecific inflammatory cytokines such as FasL generated in the implant site. Our group previously found that soluble epidermal growth factor (sEGF) promotes MSC expansion. Furthermore, tethering EGF (tEGF) onto a two-dimensional surface altered MSC responses, by restricting epidermal growth factor receptor (EGFR) to the cell surface, causing sustained activation of EGFR, and promoting survival from FasL-induced death. sEGF by causing internalization of EGFR does not support MSC survival. However, for tEGF to be useful in bone regeneration, it needs to allow for MSC differentiation into osteoblasts while also protecting emerging osteoblasts from apoptosis. tEGF did not block induced differentiation of MSCs into osteoblasts, or adipocytes, a common default MSC-differentiation pathway. MSC-derived preosteoblasts showed increased Fas levels and became more susceptible to FasL-induced death, which tEGF prevented. Differentiating adipocytes underwent a reduction in Fas expression and became resistant to FasL-induced death, with tEGF having no further survival effect. tEGF protected undifferentiated MSC from combined insults of FasL, serum deprivation, and physiologic hypoxia. Additionally, tEGF was dominant in the face of sEGF to protect MSC from FasL-induced death. Our results suggest that MSCs and differentiating osteoblasts need protective signals to survive in the inflammatory wound milieu and that tEGF can serve this function.

Abstract

Multipotential stromal cells/mesenchymal stem cells (MSCs) are attractive candidates for regenerative therapy due to the ability of these cells to differentiate and positively influence neighboring cells. However, on implantation for wound reconstruction, these cells are lost as they are challenged by nonspecific inflammation signals generated in the wound environment and in response to any implanted foreign body. We have previously shown that sustained and surface-restricted epidermal growth factor receptor (EGFR) signaling by a tethered form of its prototypal ligand EGF enhances survival of MSC in the presence of death cytokines such as FasL, serum deprivation, and low oxygen in vitro. This was proposed to be due to the plasma membrane restriction of EGFR signaling. Interestingly, during wound repair, an extracellular matrix (ECM) component Tenascin-C (TNC) containing EGF-like repeats (EGFL) and fibronectin-like repeats (FNL) is upregulated. A few of the 14 EGFL on each of the 6 arms, especially the 14th, bind as low-affinity/high-avidity ligands to EGFR causing sustained surface-restricted EGFR signaling. We queried whether signaling by this physiologically relevant EGFR matrikine also protects MSCs from FasL-induced death. MSCs grown on TNC and Collagen I (as TNC by itself is antiadhesive) displayed a survival advantage in the presence of FasL. TNC neither sequestered nor neutralized FasL; rather, the effects of survival were via cell signaling. This survival was dependent on TNC activating EGFR and downstream pathways of Erk and Akt through EGFL; to a much lesser extent, the FNL of TNC also contributed to survival. Taken together, these results suggest that providing MSCs with a nonimmunogenic naturally occurring ECM moiety such as TNC enhances their survival in the presence of death factors, and this advantage occurs via signaling through EGFR primarily and integrins only to a minor extent. This matrix component is proposed to supplement MSC delivery on the scaffolds to provide a survival advantage against death upon in vivo implantation.

Abstract

Multipotent stromal cells (MSCs) can be differentiated into osteoblasts and chondrocytes, making these cells candidates to regenerate cranio-facial injuries and lesions in long bones. A major problem with cell replacement therapy, however, is the loss of transplanted MSCs at the site of graft. Reactive oxygen species (ROS) and nonspecific inflammation generated at the ischemic site have been hypothesized to lead to MSCs loss; studies in vitro show MSCs dying both in the presence of ROS or cytokines like FasL. We questioned whether MSCs themselves may be the source of these death inducers, specifically whether MSCs produce ROS under cytokine challenge. On treating MSCs with FasL, we observed increased ROS production within 2 h, leading to apoptotic death after 6 h of exposure to the cytokine. N-acetyl cysteine, an antioxidant, is able to protect MSCs from FasL-induced ROS production and subsequent ROS-dependent apoptosis, though the MSCs eventually succumb to ROS-independent death signaling. Epidermal growth factor (EGF), a cell survival factor, is able to protect cells from FasL-induced ROS production initially; however, the protective effect wanes with continued FasL exposure. In parallel, FasL induces upregulation of the uncoupling protein UCP2, the main uncoupling protein in MSCs, which is not abrogated by EGF; however, the production of ROS is followed by a delayed apoptotic cell death despite moderation by UCP2. FasL-induced ROS activates the stress-induced MAPK pathways JNK and p38MAPK as well as ERK, along with the activation of Bad, a proapoptotic protein, and suppression of survivin, an antiapoptotic protein; the latter two key modulators of the mitochondrial death pathway. FasL by itself also activates its canonical extrinsic death pathway noted by a time-dependent degradation of c-FLIP and activation of caspase 8. These data suggest that MSCs participate in their own demise due to nonspecific inflammation, holding implications for replacement therapies.

Abstract

Multipotential stromal cells (MSCs) have been touted to provide an alternative to conservative procedures of therapy, be it heart transplants, bone reconstruction, kidney grafts, or skin, neuronal and cartilage repair. A wide gap exists, however, between the number of MSCs that can be obtained from the donor site and the number of MSCs needed for implantation to regenerate tissue. Standard methods of MSC expansion being followed in laboratories are not fully suitable due to time and age-related constraints for autologous therapies, and transplant issues leave questions for allogenic therapies. Beyond these issues of sufficient numbers, there also exists a problem of MSC survival at the graft. Experiments in small animals have shown that MSCs do not persist well in the graft environment. Either there is no incorporation into the host tissue, or, if there is incorporation, most of the cells are lost within a month. The use of growth and other trophic factors may be helpful in counteracting these twin issues of MSC expansion and death. Growth factors are known to influence cell proliferation, motility, survival and morphogenesis. In the case of MSCs, it would be beneficial that the growth factor does not induce differentiation at an early stage since the number of early-differentiating progenitors would be very low. The present review looks at the effect of and downstream signaling of various growth factors on proliferation and survival in MSCs.